There are some conventional methods for producing 2,5-dichlorophenylthioglycolic acid, including the following:
(a) the method in which 1,4-dichlorobenzene is sulfochlorinated with chlorosulfonic acid and then reduced to 2,5-dichlorothiophenol with zinc powder under acidic conditions, followed by reaction with monochloroacetic acid to yield 2,5-dichlorophenylthioglycolic acid (U.S. Pat. No. 3,440,288),
(b) the method in which 2,5-dichloroaniline is diazotized and then reacted with thiourea in the presence of copper sulfate and subsequently hydrolyzed to 2,5-dichlorothiophenol, followed by reaction with monochloroacetic acid to yield 2,5-dichlorophenylthioglycolic acid (Ger. Offen. DE3715508),
(c) the method in which 1,2,4-trichlorobenzene and sodium hydrosulfide are reacted in the presence of copper acetate catalyst in liquid ammonia solvent under increased pressure to yield 2,5-dichlorothiophenol, followed by reaction with monochloroacetic acid to yield 2,5-dichlorophenylthioglycolic acid [Kogyo Kagaku, 70, 1384 (1967)].
However, these known methods respectively have the following drawbacks.
In the method of (a) above, a large amount of waste effluent which contains harmful heavy metals poses a major problem of environmental pollution. In the method of (b) above, the copper compound used is difficult to dispose of as in (a) and the production efficiency is low; it is not an economic method for production. In the method of (c) above, liquid ammonia is difficult to handle and the copper compound used is difficult to dispose of, and additionally special equipment is required because the reaction is carried out under high pressure. Also, there is a problem of very low yield.
As stated above, all these known methods have various drawbacks and are not industrially advantageous.
For this reason, attempts have been made to develop an industrially advantageous method for producing 2,5-dichlorophenylthioglycolic acid, but there is no satisfactory method.